This invention relates to a fiber-reinforced composite. Processes are known for producing a fiber-reinforced composite by drawing fibers into a pultrusion device, impregnating the fibers with resin, and simultaneously forming and curing the structure in a heated die. (See Encyclopedia of Polymer Science and Engineering, 2.sup.nd Edition, Vol. 4, John Wiley & Sons, New York, pp. 1-28 (1986).) Inasmuch as low melt viscosity is required for efficient resin impregnation (a necessary requisite for acceptable properties of the composite), thermosets have been used in preference to thermoplastic materials. Although thermoset composites have excellent mechanical properties, they suffer from several disadvantages: Thermoset matrices have relatively limited elongation, the thermoset precursors are a source of undesirable volatile organic compounds (VOCs), the composites cannot be reshaped or recycled, and their production rates are limited.
In recent years, efforts have been directed toward making composites using thermoplastic materials. For example, Hawley in U.S. Pat. No. 4,439,387, incorporated herein by reference, teaches the extrusion of molten thermoplastic resin material through a die which imbeds the fibers. In U.S. Pat. No. 4,559,262, Cogswell et al., incorporated herein by reference, discloses a fiber-reinforced composition that is obtained by drawing a plurality of fibers continuously through an impregnation bath, which is a static melt of a thermoplastic polymer of sufficiently low molecular weight (resulting in lower melt viscosity) to adequately wet the fibers. Suitable polymers taught by Cogswell et al. include thermoplastic polyesters, polyamides, polysulfones, polyoxymethylenes, polypropylene, polyarylene sulfides, polyphenylene oxide/polystyrene blends, polyetheretherketones and polyetherketones. Cogswell et al. also teaches that in order to achieve acceptable physical properties in the reinforced composition, it is preferred that the melt viscosity be in excess of 1 Ns/m.sup.2. Thus, if the molecular weight of the thermoplastic resin is low enough to achieve sufficiently low melt viscosity to process the resin, the properties of the resultant composite suffer.
The thickness of a single ply of the fiber-reinforced sheet (or tape) is limited by the processes of the prior art. For example, Cogswell et al. teaches single-ply tape thicknesses around the order of 0.1 mm (col. 21, lines 29-31, and col. 22, lines 29-30). In order to achieve a thicker tape, several tapes have to be stacked and compression molded (col. 22, lines 33 to 48.)
In principle, thermoplastic composites would solve many of the problems associated with thermosets. For example, unlike thermosets, thermoplastics can be reshaped, welded, staked, or thermoformed. Furthermore, thermoplastics are generally tougher, more ductile, and have greater elongation than thermosets. Unfortunately, composites prepared by imbedding fibers in a typical thermoplastic resin suffer from a number of disadvantages. First, as previously noted, low molecular weight resins are required to achieve the low viscosities necessary for processability. Second, complete impregnation generally requires slow haul-through rates. Third, the static impregnation bath can cause the polymer melt to be hot for an unduly long time, resulting ultimately in polymer degradation. Fourth, the shape and size of the final composite is limited. For example, the thickness of a single ply of a thermoplastic composite tape is generally not greater than about 0.1 mm, and the length of the composite is limited to not greater than about 100 mm.
There is a need to balance processability of the thermoplastic resin with the final physical properties of the composite. It would be desirable, therefore, to have a fiber-reinforced composite prepared using a thermoplastic resin that has a sufficiently low melt viscosity to adequately wet the fiber. At the same time, it would be desirable that the resin not be limited by molecular weight restrictions as a means of achieving low melt viscosity, so that the composite prepared using such a resin exhibits improved physical properties as compared to the thermoplastic composites prepared as described in the art. It would also be an advance in the art to eliminate the static impregnation bath with an impregnation means that does not require that the melt be exposed to advanced temperatures for an unduly long time. Finally, it would be desirable to prepare longer composite tapes or articles that have single-ply thicknesses of greater than 0.2 mm, preferably greater than 0.5 mm, thereby eliminating the need of a compression-molding step to build thickness